1. Technical Field of the Invention
The present invention relates generally to an electromagnetic relay, and more particularly to an improved arc-extinguishing structure of an electromagnetic relay which is designed to bias an electric arc produced in a contact gap into a given space defined in a relay housing.
2. Background Art
A plunger type electromagnetic relay is known in the art wherein a pair of movable contacts is moved by a common movable contact retainer according to linear displacement of a plunger of a solenoid assembly.
Japanese Patent First Publication No. 59-14219, filed on Jul. 16, 1982, teaches, in FIGS. 3 and 4, an arc-extinguishing structure of a plunger type electromagnetic relay having permanent magnets and pairs of magnetic metal strips. The permanent magnets are mounted, opposite a contact gap across movable contact retainers, perpendicular to a center line of the relay in a direction offset from the center line. The pairs of the magnetic metal strips are attached to both ends of the permanent magnets, respectively, to work as magnetic poles for producing an arc-extinguishing magnetic field across the contact gap.
The above publication further discloses, in FIGS. 6 and 7, an alternative arc-extinguishing structure which has permanent magnets, mounted inside movable contact retainers, oriented magnetically in a direction perpendicular to the permanent magnets discussed above for producing an arc-extinguishing magnetic field across a contact gap.
The former structure shown in FIGS. 3 and 4 encounters the drawback in that the permanent magnets need to be fixed outside the movable contact retainers, thus resulting in an increased size of the relay in a lengthwise direction. This will lead to a bulk structure of the relay. Additionally, it is difficult to mount each pair of the magnetic metal strips so as to extend from both sides of the permanent magnet across the movable contact retainer and the contact gap. Usually, such small component parts are difficult to install in a relay housing using screws because it is inconvenient assembling operation in a narrow space. It is also infeasible to bond the magnetic metal strips to, for example, supports extending from an inner wall of a resin-made housing for the inconvenience of assembly and vibration resistance.
It is, therefore, most useful to form metal strip installation cavities in a resin-made housing in dice-casting for ease of securement of the magnetic metal strips. It is, however, difficult to form such cavities in the resin housing, especially, because it becomes difficult to remove the resin housing from a die after casting. Additionally, if the magnetic metal strips are not provided on both sides of the permanent magnets, the magnetic flux of the permanent magnets will partially act on the contact gap, resulting in reduced magnetic field across the contact gap so that an arc cannot be extinguished completely.
Further, the later structure, taught in the above publication, shown in FIGS. 6 and 7 has the permanent magnets inserted into the U-shaped movable contact retainers, respectively. This arrangement, however, assumes a decreased strength of magnetic field equal to that produced by the structure shown in FIGS. 3 and 4 from which the magnetic metal strips are omitted, thus resulting in greatly decreased arc-extinguishing ability.
U.S. Pat. No. 4,367,448 (corresponding to Japanese Patent First Publication No. 1-45688), file on Jun. 26, 1981, to Nishizako, discloses an arc-extinguishing magnetic structure of an electromagnetic contactor which has permanent magnets mounted above outer two of three contact retainers arranged in parallel to extinguish arcs with the magnetic flux produced by the permanent magnets. These permanent magnets are oriented to have opposite magnetic poles face each other to establish a magnetic flux for extinguishing an arc produced in a contact gap formed between central contacts. This arrangement is magnetically identical with that taught in the above discussed publication No. 59-14219, but different therefrom in that magnetic fluxes produced by the two permanent magnets act on one contact only.